Starch is a very commonly used material in a number of technical and industrial applications including, for example, in the production of building materials, the manufacture of paper, the treatment of textiles, the preparation of adhesives and the formulation of products such as detergent tablets or pharmaceuticals. They are also used in a variety of food applications as thickeners, binders, emulsifying agents and gelling agents, for instance.
Starch is a pseudo-crystalline material consisting of two polymers of alpha-D-glucose: amylose and amylopectin. Amylose is essentially a linear polymer in which glucose molecules are bound through alpha 1-4 bonds while amylopectin is a branched polymer containing both alpha 1-4 and 1-6 linkages.
Depending on its required use and functionality, the nature and structure of the starch molecule may have to be modified. This can be achieved by a number of techniques including thermal, chemical and enzymatic treatment.
In most applications, starch is used in the form of a gelatinised paste. Depending on the modifications performed, and on the desired end use, the starch paste will have a higher or lower viscosity. Increasing the stability of these products, and in particular of their viscosity, has been an important objective for starch producers for some time.
Viscosity stability can be improved in a number of ways including, for instance, esterification (e.g. acetylation) and etherification (e.g. hydroxypropylation or cationisation). These modifications can be carried out in the wet phase (where water is used as a vehicle for the reactants) or in the dry phase (characterised by the absence of a solvent medium).
One example of a wet process is the cationisation reaction carried out by contacting an alkaline starch slurry with a cationic agent in the presence of a gelatinization inhibitor at a temperature below the gelatinization temperature of the starch. The reaction times for such a process are characteristically between 12 and 20 hours. To obtain a starch which is also thinned—as required for certain applications—additional operations, such as acid-modification or oxidation, would have to be included in the process. The addition of such operations, however, further complicates the process and results in even greater reaction times. In any event, the reaction slurry will have to have a pH which is adjusted to a more or less neutral value and, after the reaction has been completed, it must be washed to remove salts which are used to inhibit gelatinization.
A key limitation of this wet technology is the amount of water that is wasted and the costs associated with its treatment before disposal, together with the difficulty of increasing the substitution level of starch without concurrent gelatinization.
A number of dry processes have therefore been proposed. For instance, EP0710670 (Vomm Impianti e Processi S.r.L.) describes a continuous chemical modification process by which a starch powder and a modification agent, for example a hydrolytic agent or an alkylation agent, are introduced simultaneously into a thermostatically controlled turbo-reactor comprising a propeller rotating at 300-1500 revolutions per minute. This device enables, almost instantaneously, the creation of a fluid, fine, dynamic and highly turbulent layer of a close mix between the starch particles and the chemical agent.
With such a device, the chemical modification of starch can be carried out in much shorter times. For instance, Example 1 of the patent describes the hydrolysis at 50° C. of a maize starch by hydrochloric acid wherein the retention time of the starch in the reactor is only about 30 seconds. Unfortunately, this process requires the use of a specific device which, in addition to its high energy requirements, naturally increases production costs.
To overcome these drawbacks, EP0902037 (Roquette Freres) proposes a process for thinning starch under acidic conditions. The process is continuous with a reaction time of at least 5 minutes at 60 to 100° C. The patent also discloses a method for the dry modification of starch but this involves adding a chemical modification step to the thinning process.
Thus, although it is easier to obtain thinned starch with this process than by using a specific turbo-reactor, the processing time is longer and productivity is therefore reduced. In addition, a costly heating step is necessary to bring the temperature to the required 60-100° C.
A further drawback of the process described in this patent is that, in order to produce a cationic thinned starch, multiple manipulations of the starch product are necessary (i.e. separate thinning and cationisation steps).
It is therefore apparent that an improved process for the manufacture of thinned, etherified starch is desired. It is an object of the present invention to provide such a method.